1. Field of the Invention
The present invention relates to a rangefinder optical system in which an in-focus state can be detected by superimposing two images formed by the first and second optical systems which are apart from each other by a predetermined base length.
2. Description of the Related Art
A rangefinder optical system, which is the combination of the first optical system (first window), a field frame optical system, and the second optical system (second window), is known in the art.
For example, a conventional rangefinder optical system, as shown in FIG. 6, includes the first optical system 20 through which a first image (finder image) is viewed, a field frame optical system 30 through which a field frame image is formed, and the second optical system 40 through which a second image is formed. The first optical system 20, which is formed as an inversed Galilean type finder optical system, includes a negative objective lens element and a positive eyepiece lens element 15. Through the eyepiece lens element 15, an erected image of an object with the proper orientation thereof is viewed. Furthermore, the field frame optical system 30 includes a field frame member 32 which is an opaque member on which light-transmitting portions are formed, and a reflection member 33, lens elements and the like. More concretely, one of the light-transmitting portions which is provided on the periphery of the field frame member 32 (hereinafter, the peripheral light-transmitting portion) is provided in the form of the field frame; and the other light-transmitting portion which is provided at the center thereof (hereinafter, the central light-transmitting portion) is for transmitting a bundle of rays carrying the second image. Further, in the first optical system 20, an optical-path merging prism 24 is provided, and thereby the optical path of the first optical system 20 and that of field frame optical system 30 are merged. According to this arrangement, a bundle of rays which is incident, from the object, on the peripheral light-transmitting portion of the field frame member 32 is formed as a field frame image. Subsequently, the field frame image which is superimposed onto the above first image is viewed through the eyepiece lens element 15.
The second optical system 40 includes a positive objective lens group 44 and a reflection member 41 having a plurality of reflection surfaces. The second image is formed in the vicinity of the field frame member 32. Due to the peripheral and central light-transmitting portions of the field frame member 32, the second image and the field frame image are combined thereat, and transmitted, along the optical path of the field frame optical system 30, towards the first optical system 20. The second image and the field frame image are superimposed onto the first image, and these superimposed images are viewed through the eyepiece lens element 15. Since the field frame image is required to be viewed brightly and clearly, it is preferable that the incident optical axis of the field frame optical system 30 be made parallel with the optical axis of the first optical system 20, and that the field frame member 32 be positioned to face the object. On the other hand, in order to increase the precision of the rangefinder, it is necessary to maintain the base length L between the incident optical axis of the first optical system 20 and the incident optical axis of the second optical system 40 as long as possible.
The base length L is substantially determined by the sum of the following distances:
(i) the distance from the incident surface of the second optical system 40 to the objective lens group 44;
(ii) the focal length of the objective lens group 44; and
(iii) the distance from the field frame member 32 to the first optical system 20.
The focal length of the objective lens group 44 is determined so that the finder magnification of the first optical system 20 is the same as that of the second optical system 40 in order to make the size of the images (i.e., the first image and the second image) equal. The finder magnification is defined as the angular magnification. On the other hand, if an attempt is made to avoid increasing the size of the reflection member 41, there is a need to make the distance from the incident surface of the second optical system 40 to the objective lens group 44 as short as possible. Therefore in order to maintain the longer base length L, it is necessary to increase the length P (in the direction of the base length L) between the first optical system 20 and the field frame member 32.
However, in the above conventional arrangement in which the second image is formed in the vicinity of the field frame member 32, the base length L cannot be made longer due to the following requirements to be satisfied at the same time:
(i) in order to view the field frame image clearly, the field frame member 32 has to be positioned to face towards the object so that much light from the object is incident thereon; and
(ii) for the purpose of obtaining substantially the same diopters, through the eyepiece lens element 15, for the object image formed through the first optical system 20, for the field frame member 32, and for the second image formed in the vicinity of the field frame member 32, the field frame member 32 and the first optical system 20 have to be positioned closer, since there is a need to make the position of a virtual image formed by the objective lens element of the first optical system 20, and the position of the field frame member 32 optically equivalent with respect to the eyepiece lens element 15.
In another conventional rangefinder optical system, as shown in FIG. 7, in order to make the distance between the field frame member 32 and the first optical system 20 longer, the optical axis from the optical-path merging prism 24 to the second optical system 40 is inclined. However, the field frame member 32 cannot be positioned to face towards the object, and therefore the field frame image becomes unclear.
Furthermore, in the case where the above-described finder optical system is provided in a camera, it is preferable that the dimensions of both the field frame optical system 30 and the second optical system 40 measured in a direction parallel with the optical axis of the first optical system 20 (hereinafter, the thickness of a camera) be as small as possible in order to achieve miniaturization of the camera, and to make space for providing other components. However, as explained, in front (the object side) of the field frame member 32 of the field frame optical system 30, the second optical system 40 is positioned; and in the second optical system 40, there is a need to provide an optical system to make the incident optical axis to be incident on the field frame member 32 substantially parallel with the incident optical axis of the first optical system 20. As a result, it is difficult to reduce the thickness of the field frame optical system 30 and the second optical system 40
It is an object of the present invention to provide a rangefinder optical system through which a bright and clear field frame image can be viewed, and in which the base length can be made longer.
Further, it is another object of the present invention to provide a rangefinder optical system which can reduce the thickness of the field frame optical system and that of the second optical system.
The present invention is based on a conception that the base length between the first optical system and the second optical system is made longer by merging the optical path of the field frame optical system and that of the second optical system on the side of first optical system with respect to the filed frame member, and thereby the thickness of the field frame optical system and that of the second optical system are reduced.
In order to achieve the above-mentioned objects, there is provided a rangefinder optical system including a first optical system through which a first image is viewed; a field frame optical system, having a field frame member, for forming a field frame image; and a second optical system which forms a second image, and has an incident optical axis being apart from an incident optical axis of the first optical system by a predetermined base length.
By merging the optical paths of the first optical system, the field frame optical system and the second optical system, the first image, the field frame image and the second image are viewed in a single field of view.
In the above rangefinder optical system, there are further provided a field frame reflection member for deflecting a bundle of rays transmitted through the field frame member towards an optical-path merging member in the first optical system; a second optical system reflection member (hereinafter, a second reflection member) for deflecting the incident optical axis of the second optical system towards the field frame reflection member, and an optical-path merging member of the second optical system (hereinafter, a second optical-path merging member), integrally formed on the field frame reflection member, for deflecting a bundle of rays from the second reflection member towards the first optical system, and merging the bundle of rays into the field frame optical system.
According to the above arrangement, the second image by the second optical system is formed on the side of the second reflection member with respect to the second optical-path merging member. In other words, the second image is formed at a position substantially optical-equivalent to the position of the field frame member with respect to the eyepiece lens element 15.
The rangefinder optical system according to the present invention preferably satisfies the following conditions:
|xcex8F| less than 20xc2x0xe2x80x83xe2x80x83(1)
100xc2x0 less than xcex8M less than 160xc2x0xe2x80x83xe2x80x83(2)
wherein
xcex8F designates an angle between the optical axis of the field frame optical system in the vicinity of the field frame member, and the incident optical axis of the first optical system; and
xcex8M designates an angle between the optical axis extending from the second reflection member to the second optical-path merging member, and the optical axis reflected therefrom.
The second reflection member may include, for example, a first reflection sub member of the second optical system (hereinafter, a first reflection sub member) for deflecting the incident optical axis of the second optical system towards the first optical system, and a second reflection sub member of the second optical system (hereinafter, a second reflection sub member) for further deflecting the optical axis deflected by the first reflection sub member towards the second optical-path merging member. In this arrangement, the second reflection sub member preferably satisfies the following condition:
100xc2x0 less than xcex8D less than 160xc2x0xe2x80x83xe2x80x83(3)
wherein
xcex8D designates an angle between the optical axes incident on and reflected by the second reflection sub member of the second optical system.
Further, either the first or the second reflection sub member can be constituted by a roof reflection surface having two orthogonal reflection surfaces.
The rangefinder optical system according to the present invention, the field frame member can be positioned perpendicular to the optical axis of the first optical system, and thereby a bright and clear field frame image can be obtained.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-11-55334 (filed on Mar. 3, 1999) which is expressly incorporated herein by reference in its entirety.